FIG. 19 illustrates an example of a conventional development device employed in a copying machine as an example of an image forming apparatus.
The above-mentioned development device includes a casing 80 having a rectangular shaped opening section 90 formed so as to face a photoreceptor 81, and a toner supply opening 83 formed on the casing 80.
Inside the casing 80, a developer roller 84 is provided in a vicinity of the photoreceptor 81. The developer roller 84 is composed of a fixed magnet roller 84a and a sleeve 84b rotatably mounted around the outer surface of the fixed magnet roller 84a. The sleeve 84b of the developer roller 84 is driven so as to rotate in a counterclockwise direction. On the surface of the sleeve 84b, a developer forms a layer having a brush-like surface (referred to as magnetic brushes hereinafter) as the developer is attracted magnetically by the magnetic force of the magnet roller 84a inside the developer roller 84. A doctor 85 for regulating the height of the magnetic brushes formed by the developer on the developer roller 84 is provided above the developer roller 84.
Further, inside the casing 80, in addition to the developer roller 84, first and second agitating rollers 86 and 87 are arranged in this order in a direction from the developer roller 84 to the toner supply opening 83. A transport magnet roller 88 is provided above the first and second agitating rollers 86 and 87, and a guiding plate 89 is provided between the transport magnet roller 88 and the first agitating roller 86.
The guiding plate 89 is provided such that the upper end portion thereof is positioned in a vicinity of the doctor 85, and the lower end portion thereof is inserted in the developer between the first agitating roller 86 and the second agitating roller 87. The developer regulated by the doctor 85 is guided to the lower end side of the guiding plate 89 along the upper surface of the guiding plate 89. The guiding plate 89 also plays a role in regulating the amount of the developer flowing into the side of the first agitating roller 86 through the clearance formed between the lower end of the guiding plate 89 and an inner wall surface 80a of the casing 80.
Additionally, the first and second agitating rollers 86 and 87 and the transport magnet roller 88 are driven so as to rotate in a synchronized motion in directions indicated by the arrows in FIG. 19 by a motor (not shown).
According to the above arrangement, toner, supplied through the toner supply opening 83, is agitated with the developer by the second agitating roller 87. As a result, the toner is charged in the opposite polarity to an electrostatic latent image formed on the photoreceptor 81 due to the friction with carrier in the developer. A portion of the developer including the charged toner is drawn back again by the transport magnet roller 88 along the guiding plate 89. The rest of the developer flows towards the first agitating roller 86 where it is further agitated.
The developer forwarded to the developer roller 84 is attracted magnetically to the sleeve 84b so as to form thereon the magnetic brushes. Here, the doctor 85 regulates the amount of the developer such that only a required amount of the developer is transported to the side of the photoreceptor 81 by the rotation of the sleeve 84b, then the electrostatic latent image formed on the photoreceptor 81 is developed.
In recent years, there has been a demand in the market for further decreasing the running cost. In order to meet such demand, an attempt has been made to achieve a longer life of the carrier of the developer by reducing a mechanical stress on the developer in the development device such as above, thereby reducing the running cost. However, reduction in the mechanical stress on the developer inevitably causes a decrease in the amount of charge of the toner. The reduction in the amount of charge of the toner generates another problem in that the toner scatters. For this reason, a quality copying machine that can meet the described demand in the market has not been realized.
An explanation of such case will be given referring to the development device shown in FIG. 19. In order to reduce the mechanical stress on the developer in the development device, firstly, the amount of the developer transported by the sleeve 84b is reduced so as to reduce the amount of the developer flowing into a space between the guiding plate 89 and the developer roller 84. Secondly, it is arranged such that the developer transported by the sleeve 84b is less likely to be held between the guiding plate 89 and the developer roller 84. As a result, the mechanical stress on the carrier can be reduced since a pressure is not applied to the developer.
However, the described arrangement has a drawback in that charging of the toner, as achieved by the friction between the carrier and toner of the developer cannot be expected in the state where the developer is held between the developer roller 84 and the guiding plate 89. For this reason, the amount of charge of the toner is inevitably reduced. Although the amount of charge of the toner can be increased by agitating at high speed, such high speed agitation also increases the stress on the developer.
As a result, as mentioned above, an attracting force between the toner and the carrier decreases, and an amount of a floating toner increases as a consequence. During the operation of the device, an internal pressure of the casing 80 increases due to the wind in the device generated by the rotations of the developer roller 84 and the first and second agitating rollers 86 and 87, etc., and also due to the developer flow. Consequently, the highly compressed air blows off through the opening section 90 formed on the casing 80, and the scattering of the toner occurs as the floating toner is caught in the air.
Generally, the developer does not adhere to the portions at the both ends of the developer roller 84 in the rotation axis direction. Consequently, the clearance formed between the surface of the developer roller 84 and the opening section 90 widens at the both end portions. As a result, the scattering of the toner from the both end portions of the opening section 90 in the longitudinal direction is more noticeable. The scattering of the toner in the described manner is further aggravated by a vacuum space C (refer to FIG. 19) formed in front of the doctor 85 by an air-flow rotating in response to the rotation of the developer roller 84. Additionally, the central portion of the opening section 90 has an arrangement where the air-flow rotating in response to the rotation of the developer roller 84 forms a laminar flow at the central portion in the longitudinal direction such that the floating toner is easily drawn back to the inside of the casing 80. Therefore, the amount of the scattering of the toner at the central portion is small compared with that at the side of the end portions.
Further, according to the described arrangement having the guiding plate 89, the upper end side of the guiding plate 89 is clogged with the developer regulated by the doctor 85. On the other hand, since the lower end portion of the guiding plate 89 is inserted in the developer, two spaces A and B parted by the guiding plate 89 are formed. As a consequence, the internal pressure of the space A where the opening section 90 is located becomes particularly high, thereby increasing the amount of the toner scattering through the opening section 90.
Meanwhile, methods for suppressing the scattering of the toner are disclosed in Japanese Unexamined Utility Model No. 142962/1989 (Jitsukaihei 1-142962), Japanese Unexamined Utility Model No. 196262/1986 (Jitsukaisho 61-196262), Japanese Unexamined Patent publication No. 139587/1990 (Tokukaihei 2-139587), Japanese Unexamined Patent publication No. 145570/1986 (Tokukaisho 61-145570), Japanese Unexamined Utility Model No. 170953/1987 (Jitsukaisho 62-170953), Japanese Unexamined Patent publication No. 244072/1987 (Tokukaisho 62-244072), and Japanese Examined Patent publication No. 39581/1989 (Tokukohei 1-39581).
1 Japanese Unexamined Utility Model No. 142962/1989 (Jitsukaihei 1-142962)
According to the method for suppressing the scattering of the toner disclosed in this publication, in the case where two spaces are formed, in which a pressure increases and decreases respectively in response to the rotation of the developer roller, inside the casing, vents having filters are formed on the wall around a development tank encasing each space. The pressure difference between the inside and the outside of the development tank is equalized by connecting the each space to the outside, thereby suppressing the scattering of the toner.
2 Japanese Unexamined Utility Model No. 196262/1986 (Jitsukaisho 61-196262)
According to the method for suppressing the scattering of the toner disclosed in this publication, a pressure in the casing is reduced by providing connecting means having a filter for connecting a space having a high pressure surrounded by the developer roller, the agitating rollers, and the casing, to the outside of the casing, thereby suppressing the scattering of the toner.
3 Japanese Unexamined Patent Application No. 139587/1990 (Tokukaihei 2-139587)
According to the method for suppressing the scattering of the toner disclosed in this publication, by providing an air flow guiding plate at the lower end side of the opening section formed on the casing as well as vents (pressure releasing pores) having filters on the casing, the pressure inside the casing is reduced, thereby suppressing the scattering of the toner.
4 Japanese Unexamined Patent Application No. 145570/1986 (Tokukaisho 61-145570)
According to the method for suppressing the scattering of the toner disclosed in this publication, in a development device having seal materials for sealing the clearance formed between the photoreceptor and the edge of the opening section formed at the upper end side of the opening section formed on the casing, vents (pressure releasing pores) having filters for suppressing a pressure increase inside the casing are formed in a vicinity of the doctor attached to the casing, thereby suppressing the scattering of the toner.
5 Japanese Unexamined Utility Model No. 170953/1987 (Jitsukaisho 62-170953)
According to the method for suppressing the scattering of the toner disclosed in this publication, by forming vents (air intake pores) for taking in the outside air in a vicinity of the both end portions of the developer roller in a rotation axis direction, and by forming a predetermined clearance between the edge portion of the opening section formed on a casing and the outer surface of the photoreceptor, an air-flow in a direction from the outside of the development device to the inside is generated, thereby suppressing the scattering of the toner.
6 Japanese Unexamined Patent Application No. 244072/1987 (Tokukaisho 62-244072)
According to the method for suppressing the scattering of the toner disclosed in this publication, by releasing the pressure inside the casing, and by setting a clearance between the edge portion of the opening section formed on the casing and the surface of the developer roller according to the thickness of a developer layer formed on the developer roller, an air-flow is circulated inside the casing, and the air is prevented from blowing off as a result, thereby suppressing the scattering of the toner.
7 Japanese Examined Patent Application No. 39581/1989 (Tokukohei 1-39581)
According to the method for suppressing the scattering of the toner disclosed in this publication, an air outlet is formed below the developer roller, and an air flow guiding plate is formed in a shape such that the clearance between the air flow guiding plate and the developer roller widens in a direction from the opening section of the casing to the downstream side of the developer roller so as to guide an air-flow to the air outlet such that an air-flow is generated so as to be sucked into the casing through the opening section, thereby suppressing the scattering of the toner.
Here, in the following explanations in the specification, if not specified, the scattering of the toner suggests that from the opening section, similarly, "the side portion" and "the central portion" etc. suggest those in the rotation axis direction of a developer holder (developer roller), and in the longitudinal direction of the development device.
However, according to the methods disclosed in the described publications 1 through 5, the scattering of the toner can be suppressed, yet they have problems of a difficulty in maintenance, an increase in cost, the toner passing through the filter, and a deterioration of the image due to a black lead. Further, according to the methods disclosed in the above-described 6 and 7, the scattering of the toner is not suppressed uniformly in the rotation axis direction of the developer roller.
Namely, according to the methods of 1 and 2, a portion in the casing having the highest pressure is connected to the outside air which requires a component such as a filter, etc., for reducing the pressure in the portion having a high pressure, thereby increasing the number of components and the cost. Further, since the air in the portion having a high pressure directly passes through the filter, the life of the filter is shortened, and a frequent replacement of the filter becomes necessary. Consequently, maintenance of the development device becomes difficult. Further, in the case where the air in the portion having a high pressure passes through the filter, the possibility of the floating toner passing through the filter increases, also the inside of the image forming apparatus is contaminated as the toner scatters out of the development device.
According to the methods disclosed in the publications 3 and 4, a portion in the casing having a pressure, although not the highest, is connected to the outside air so as to reduce the pressure in the casing. Therefore, as in the methods of 1 and 2 described above, the floating toner tends to pass through the filter, and it is required to replace the filter in a short cycle. The scattering of the toner tends to occur especially in the case where the pressure is suddenly increased immediately after the developer roller and the agitating rollers are started to be driven.
According to the method disclosed in the publication 5, the scattering of the toner is suppressed by forming vents for taking in the outside air in a vicinity of the both end portions of the developer roller. However, according to this arrangement, since the air is taken in from seal materials at the both ends, the adhesion of the toner to the front portion of the doctor cannot be prevented, so that the toner adheres to the doctor. The toner adhering to the doctor falls in the form of a mass, thereby generating the black lead on the image which lowers the quality of the image.
According to the methods disclosed in the publications 6 and 7, the lower end side of the opening section formed uniform on the casing has a uniform shape to the entire length in the longitudinal direction. Nevertheless, the developer does not adhere to the surface of the roller at both end portions of the developer roller. Further, since the developer does not adhere to the developer roller, the air is likely to be blown out of the inside of the casing. Moreover, a magnetic force of the developer roller decreases at the both end portions of the developer roller. The 6 and 7 fail to refer to such problem that directions of the air-flow around the developer roller differ between the central portion and the both end portions of the opening section, which causes a difference in the pattern of the scattering of the toner, thereby presenting a problem that the force of suppressing the scattering of the toner is not applied uniformly.